Despite numerous advances over the years, there exists a continuing need for improved liquid purification. In my copending application, allowed U.S. Ser. No. 08/925055, which is incorporated herein by reference, I describe a novel reverse osmosis system in which a plurality of reverse osmosis membranes are placed within a well or other underground channel. The channel contains an unpurified liquid such as salty or brackish water, and is deep enough that overhead pressure cooperates with the membranes to select out a permeate. Such a system greatly reduces the amount of liquid that needs to be pumped, but still requires raising the permeate from a significant depth, which in the case of processing salt water may be approximately 1800 feet.
It is known to raise a liquid from a well or other underground channel using a series of positive pressure mechanical pumps. That solution, however, is problematic in that one or more of the pumps may fail during operation, and may therefore require raising of the entire chain of reverse osmosis membranes to the surface to effect repairs. Placing pumps below the ground also requires providing an electrical or other power line to the pumps, which adds complexity, cost, and further opportunities for failure.
It is therefore desirable to locate the pumping mechanism at or above ground level. One way of accomplishing that goal is to use an air lift pump. Air lift pumps have been known for at least 200 years. As exemplified by U.S. Pat. No. 4,917,832 to Marcum et al., air lift pumps generally include a source of compressed gas such as oxygen or air, a tube which carries the compressed gas to a diffuser at a subsurface level, a means for mixing the compressed air exiting the diffuser with the liquid to be raised, and a plurality of riser pipes in which the liquid and gas are permitted to rise. It is known that the use of relatively larger bubbles, especially those which extend completely across the interior of the inner riser pipe, tends to increase the volume of liquid pumped.
Liquids rising under the force of an air lift pump may exit the riser pipes with considerable force. Such water may be deflected with a deflector, and it may also be possible to harness some of the kinetic energy contained in the rising liquid by discharging such liquid through a turbine near the top of the risers. See, e.g., U.S. Pat. No. 4,135,364 to Busick et al.